Green’s Dictionary of Slang

say adv.

also sey
[backsl.]

yes.

[UK]Hotten Dict. of Modern Sl. etc.
[UK](con. 1840s–50s) H. Mayhew London Labour and London Poor III 208/1: If it is, I say, ‘Say.’ That is, – Yes. If not, I say ‘Nanti’.
[UK]Hotten Sl. Dict.
[UK]Northampton Mercury 12 Apr. 12/4: ‘Shif,’ fish; ‘torrac,’ carrot; ‘yad,’ day; ‘kool,’ look; ‘sey,’ yes; ‘on’, no; ‘pac,’ cap; ‘reeb,’ beer, ‘yob,’ boy; are all perfect specimens [i.e. of backslang].
[UK]J. Franklyn Cockney 298: Had the manager replied Say (Yes) instead of On (No) the customer would have gone away happy.
[NZ]W. Ings ‘Trolling the Beat to Working the Soob’ in Int’l Jrnl Lexicog. 23:1 59: Backslang is a language form that writes or pronounces words backwards, for example, ecaf (face), riah (hair), say (yes), yob/yobbo (boy).